1. Field of the Invention
The invention relates to a method for decentralized controlled transmission rerouting of operating paths in a message transmission network comprising operating stations, restoration links, switching matrices and rerouting control devices.
2. Prior Art
Message transmission networks enhance their reliability by use of automatic rerouting switching for individual message transmission paths in case of faulty interruption of transmission paths. When assigning restoration links, two different categories can be used, the direct transmission restoration system and the transmission rerouting system.
In the transmission rerouting system (decentralized concept) N rerouting links are permanently assigned to M operating paths between two line ends (terminal stations of the transmission path). German Pat. No. DE-AS 27 53 420 discloses an arrangement for direct transmission restoration system for digital signals, in which the fifth to seventh bit of the message word are substituted by the address of the transmission system to be switched over. From a line terminal station in the substituted system this address is returned, in a combination not used for addresses, as an acknowledgement signal to the terminal station in the substitute system which transmits the address. To monitor the individual transmissions sections the frame synchronization word is used. The alarm is made known by means of a signal commonly referred to an alarm indication signal (AIS), which is produced by an AIS generator arranged in the line terminal station and which is used to suppress trailing alarm signals on the subsequent signal paths. Hence, the occurrence of alarm signals is avoided in all transmission sections subsequent to a disturbed transmission section.
FIG. 1 shows the message transmission network for such a method of the direct transmission restoration. Also in a meshed network only the control devices S assigned to the line end terminals LE can dispose of the N restoration links E. In case of disturbances affecting more than N normal links, up to M-N links will not have the possibility of transmission restoration.
The monitoring of the transmission links and the switching of the normal links B to the restoration links E is controlled by the control devices S1, S2 arranged at the line ends. If the substitute circuit has to be switched to for both directions of transmission (including the possibly undisturbed opposite direction), an exchange of data will be required between the control devices S1, S2 aranged at the line ends. One of the control devices S1 assigned to the line terminal stations LE has to assume the master function for both directions of transmission.
With a 1:1 (that is to say M=N=1) direct transmission restoration there is no need to exchange data between the control devices S, as restoration link E and normal link B simultaneously carry the operating signal and, consequently, only have to be switched over in case of a failure.
With a transmission rerouting in a meshed transmission network there is a possibility, to switch to more than restoration paths E replacing M normal paths B, when there are N&lt;M restoration links E between the restoration switching points (restoration switching matrices), as long as idle restoration links E are still available in the network. This is achieved by bypassing on a larger scale the failed normal links B, which allows switched to further restoration links E in addition to the N restoration links E directly assigned. With respect to such transmission rerouting, so far only a centrally controlled concept has been implemented, which implies that the transmission rerouting is controlled by the central station M. FIG. 2 shows the network structure for a centrally controlled transmission rerouting.
In order to suppress trailing alarm signals, for example when signal loss occurs, the alarm indication signal AIS is used in digital transmission technology, which signal is a permanent "1" signal in the digital hierarchy building up to 2.048 Mbit/s. In the European Patent Application EP-B 10 31 943 a monitoring of transmission paths for digital signals is disclosed, in which an AIS generator arranged in the line terminal station LE produces a signal, which is used to suppress trailing alarm signals on the subsequent signal path. Thus, it is avoided that in case of disturbance in a transmission path section, alarm signals will occur in all subsequent sections of the transmission paths. In the centrally controlled rerouting system disclosed in EP-B-10 031 943 such AIS generators are also arranged in the intermediate regenerators. The line terminal stations LE comprise monitoring devices in addition to the AIS generators, which devices are connected to the central network monitoring station M via a control line D. All disturbances are reported in first instance to the central station M, and this central station M gives the relevant orders to the substitute rerouting controls KF1, KF2, . . . involved, on the basis of the condition of its information about the map of the network. From the central station M the AIS generator can also be connected, remote-controlled via a change-over switch, to the output of the line terminal station LE on the receiving side.
Conflict situations are impossible with the centrally controlled transmission rerouting system, even when fault messages from individual sections arrive at the central station M for both transmission directions uncoordinated in time and at different instants. In the central station M the actual map of the network that is used for controlling the substitute system is stored continuously.
The method of centrally controlled transmission rerouting described above has the disadvantage, that essentially two operations for the transmission of information are required, that is to say a message about the interruption to the central station M and the transmission of the control message to the substitute switching controls KF1, KF2 . . . , which also require extra time.